(1) Field of the Invention
The present invention relates to a malfunction detector for detecting malfunction in a machine.
(2) Description of the Prior Art
Malfunction detectors are known for detecting malfunctions in machines, such as punch presses or die stamping machines. Switches are mounted in such machines so as to be actuated for turning on or off in response to certain operations. In order to safeguard the machine, for instance so as to prevent it from damaging itself should a malfunction occur, such a malfunction detector monitors the state of the switches and is arranged to recongnize any invalid state so as to warn an operator and/or automatically stop the machine.
The switches applied to such machines normally have one pole connected to the machine frame and the other pole connected to the known malfunction detector. A direct current is injected into the frame of the machine and the malfunction detector detects the passage of direct current via any switch which is closed so as to monitor the switching state of the switches. However, voltage surges and increasing or decaying alternating current waves have direct current components and this leads to the problem that interference from such sources can be caused by voltages being induced into the sensing lines between the switches and the malfunction detector from fluctuating voltages and currents in adjacent circuits and in the power supply to the machine. The input stages of the malfunction detector, therefore, have to be designed so as to be insensitive to such interfering signals, but nevertheless interfering signals of large magnitude can still cause incorrect sensing of the states of the switches, leading to incorrect operation of the malfunction detector.
Problems can also be encountered with the switches themselves. Because the test signal applied to them is a direct current, the contacts may be susceptible to electrolysis. Also, the switch contacts may be subjected to contamination, for instance by lubricating oil, cooling liquid, and metal particles. Thus, the resistance of the switches can change with time so that the theoretically zero resistance when the switch is closed can become a significant resistance value and the theoretically infinite resistance when the switch is open can be reduced to a finite value which cannot be ignored. Since the input circuits of known malfunction detectors check for open and closed switch states, the degradation with time or the switching states can cause spurious and incorrect readings, again leading to incorrect operation of the malfunction detector.
In cyclic machines, i.e., machines which repeat a cycle of operations over and over again, it is usual for the state of certain switches which monitor the cyclic operation to be tested during a specific time period related to the cycle or operation. In other words, such switches are monitored only during a predetermined portion of each cycle of operation. The known malfunction detectors respond to the switching state of the switches and it is, therefore, possible for erroneous detection to occur. In particular, if one of the switches fails in its state indicating that operation of the machine is correct, than this switch will be monitored during each monitoring period and the malfunction detector will indicate that operation is correct when, in fact, the switch is no longer monitoring the operation. The detector may, therefore, fail to detect a malfunction, and this can lead to damage of the machine, damage of a part being produced or acted upon by the machine, and possibly injury to personnel.